1. Field of the Invention
The present invention relates to a camera system having a signal communication function between a camera body and a photographing lens and an improved photographing lens thereof.
2. Description of Related Art
In a recent single lens reflex camera having an automatic focusing device, information peculiar to a photographing lens, such as open F-number data used in an automatic exposure function or an automatic focusing function, is sent as electrical signals from the photographing lens to a CPU (central processing unit) associated with a camera body.
Information peculiar to the photographing lens is stored in a lens ROM provided in the photographing lens. The photographing lens and the camera body are provided with electrical contacts on a lens mount and a body mount to transmit and receive the signal information between the photographing lens and the camera body, respectively. The body CPU transmits and receives the signal information to and from the lens ROM through the electrical contacts and reads the data stored in the lens ROM. A clock pulse is output from the camera body to send address signals from the body CPU synchronously therewith, so that predetermined data can be read out from the lens ROM in accordance with the address signals. The reading of the data from the lens ROM is carried out in accordance with a serial communication that is controlled by the body CPU. The body CPU can not simultaneously perform other operations in real time during this communication.
Furthermore, in the case of a photographing lens having an AF motor (automatic focusing motor) and a PZ motor (power zoom motor), etc., if the control of these motors is affected by the body CPU, there is an overload on the body CPU. In particular, in a single lens reflex camera in which various kinds of photographing lenses are exchangeably mounted to one camera body, the body CPU must read different parameters, depending on the photographing lenses, from the associated lens ROMs and perform predetermined arithmetic operations in accordance with the read parameters so as to control the photographing lenses. This makes quick operations impossible, or next to impossible.
On the other hand, the provision of different lens ROMs for every photographing lens increases the manufacturing cost thereof.
It is desirable in a single lens reflex camera that if a novel camera body or photographing lens is newly developed, the new camera body or the new photographing lens can be used for an old type photographing lens or camera body, and vice versa.